What gets some pets excited: roughly rising quantity of milk? (6)
I believe the answer is:
catnip
'what gets some pets excited' is the definition.
(substance which excites cats)
'roughly rising quantity of milk?' is the wordplay.
'roughly' becomes 'ca' (abbreviation for circa).
'rising' is a reversal indicator (going up in a down clue).
'quantity of milk?' becomes 'pint' (I can't explain this - if you can you should give a lot more credence to this answer).
'pint' back-to-front is 'tnip'.
'ca'+'tnip'='CATNIP'
Can you help me to learn more?
(Other definitions for catnip that I've seen before include "treat for moggie" , "Feline attractor (I, not E)" , "Hairy aromatic perennial herb strongly attractive to some pets" , "Kitty likes that!" , "feline favourite?" .)